BR0309056B1 - polycrystalline alumina refractory material and use thereof. - Google Patents

Abstract

The invention concerns a refactory material based on corundum treated by electrofusion consisting of beads of diameter ranging between 50 mum and 5 mm, having a total pore ratio ranging between 10 and 50 % of the volume of the beads, the sealed porosity representing from 80 to 98% of the total porosity. The inventive material is in particular designed for spray-applied refractory concrete.

Description

Report of the Invention Patent for "Polycrystalline Alumina-Based Refractory Material and Use".

Technical Field of the Invention The invention relates to alumina-based refractories intended notably for the most diverse furnace coatings, as well as for the casting of melting crucibles, capable of receiving melt materials. State of the Art

The use of alumina refractories is widespread, for example in the form of white fused corundum or in the form of high temperature calcined alumina in the rotary kiln. Preparation of an alumina suitable for a refractory application does not remain less delicate operation. The raw material consists essentially of bauxites, natural alumina hydrates from which alumina is extracted by alkaline attack to form a sodium aluminate, from which the alumina hydrate is subsequently precipitated, which then gives calcination alumina.

However, the calcination of aluminum hydroxide is known to be done by a sequence of complex transformations, which succeed to high temperatures. Thus, to obtain a stable material, it is convenient to calcine the product at very high temperature. It is the choice that is made in the case of said tabular alumina.

Since the temperature should be raised a lot, one might prefer to go to the melt; The electrofused corundum is then prepared whose use as a refractory material is also well known.

In both cases, the alumina should be brought to a suitable particle size. In the case of tabular alumina, the material is bolted before calcu- lation, while in the case of electrofused corundum the material is assembled and screened.

Direct shaping attempts have been made in the past. Particular mention may be made of US Patent 1,871,793 to Alcoa, 1932, which describes the atomization of the fused alumina into hollow spheres of less than 5 mm in diameter and a wall thickness of less than 250 μm. . The product obtained is a metallurgical alumina, intended to serve as a raw material for the manufacture of aluminum; its use as refractory is not considered. Indeed, it is likely that this material made up of spheres with ping-pong miniballs must have very poor mechanical strength.

On the other hand, atomized electrofused corundum is commonly manufactured to produce a material in the form of filled spheres which are then used for the manufacture of abrasives.

Purpose of the Invention

The purpose of the present invention is to provide an alumina-based refractory material having good thermal stability and good compressive strength, usable notably in the form of refractory concrete to be designed.

The invention is directed to an electrofused cast-iron refractory material consisting of spheres of a diameter of 50 μm to 5 mm, having a total porosity rate of 10 to 50% of the spherical volume, the closed porosity representing 80 to 98% of total porosity. Description of the Invention

When manufacturing abrasive-atomized electrofused corundum balls, the applicant found at the time of manufacturing incidents that intermediate products could be obtained between filled and hollow balls. The material looks good in the form of spheres, but the structure of these spheres is that of a more or less dense sponge; whereas the hollow sphere has a single sphere-centered macroporosity, the material obtained has numerous macroporosities of any shape, more or less evenly distributed. At the end of multiple tests to reduce the open porosity of these spheres, the applicant has even defined operating conditions, allowing spheres to be obtained in which few porosities discharge to the surface.

The material according to the invention has a higher compressive strength with respect to the hollow sphere, while its thermal conductivity remains low due to the presence of porosities.

It is in the form of substantially spherical spheres having an external diameter of between 50 μιη and 5 mm, and preferably between 0.5 mm and 2 mm. These spheres have a mostly closed porosity, comprised between 10 and 50% of the total volume of the balls, and preferably between 20 and 30%. The open porosity, corresponding to the pores that flow into the sphere surface, measured by pycnometry of water under atmospheric pressure, is between 2 and 20%.

The material of these spheres is made up of polycrystalline alumina formed from crystallites less than 50 μητι. This structure is remarkably stable up to about 1500 ° C. It can be verified by examining the material under the electron microscope after annealing at 1500 ° C, and this examination shows that no change can change the structure of the product.

The compressive strength of the balls according to the invention is typically between 150 and 300 g, against 30 to 40 g for the prior art hollow balls, with a thermal conductivity of the bulk material in the field of temperature 200 ° C - 1200 ° C 0.5 to 0.8 Wm "10K" 1, against 0.4 to 0.6 Wm "10K" 1 for the prior art hollow spheres.

This material allows, in particular, the preparation of easy-to-use projecting concretes, the property of the spherical shape of the particles of which it is made.

According to the invention, the manufacturing process of the products consists in the flow of white or brown blown-corundum onto a horizontal air-laden sheet, which leads to atomization of the product. The most suitable parameters for obtaining the products according to the invention are as follows: - air pressure on the blade: 0.05 to 0.1 MPa;

- blade width: 2 mm per kg of liquid corundum to be treated in

one minute;

- blade thickness: few millimeters;

- ratio of water flow to molten corundum flow rate: 0.2

1.5 liters of water per kg of molten corundum. Examples Example 1

It was melted into a 2 MW arc furnace of Bayer alumina in sufficient quantities to have a reserve of approximately 2 tonnes of electrofused white corundum. This reserve allowed the continuous flow of a regular liquid corundum jet for 10 minutes at a rate of 100 kg per minute directly over a mixed air and water slide.

This blade was generated by a 240 mm wide and 6 mm high rectangular section orifice, positioned in a vertical plane, and fed by an air source, maintaining a constant pressure of 0.09 MPa over the orifice. . The airstream received an injection of 22 kg of water per minute.

The corundum thus fused was recovered in a pit downstream of the installation and its particle size was as follows:

- fraction <0.5 mm: 15%;

- fraction> 2 mm: 20%

- fraction> 5 mm: 3%

The product porosity rate was 28%, of which 1.3% open porosity.

The compressive strength of the spheres was 260 g.

Example 2

Refractory concrete was prepared from a mixture of hollow products of the following weight composition:

- brown corundum of particle size 2 to 5

mm: 23%; - brown corundum of particle size <0,2 mm: 24%

- amorphous microsilica in spheres of size less than 20 µm: 5%

- Lafarge low alumina P622B alumina Aluminates: 5%

For compression and placement, the following products were added to the mixture:

- sodium metaphosphate: 0.2%

- Darvan C ® ammonium polymethacrylate from Polyplastic: 0.3%

- water: 6%

After drying and firing, a refractory concrete of 3.0 kg / dm3 density was obtained.

Its thermal conductivity measured at 600 ° C was 5.5 W ° m "1oK" 1, and its compressive strength was estimated at 110 MPa. Example 3

Refractory concrete was prepared from a dry product mixture of the following weight composition:

- spheres according to the invention of a size between 2 and 5 mm: 15,4%

spheres according to the invention of a size between 0.2 and 2 mm: 37%

- brown corundum of size <0,2 mm: 30%

- amorphous microsilica in spheres <20 μιτι: 5.5% - alumina P622B: 6.6%

- Drying cement 80: 5.5%

For compression and placement, the following products were added:

- sodium metaphosphate: 0.2% - Darvan C®: 0.3%

- water: 8%

After drying and firing, a refractory concrete of 2.5 kg / dm3 density was obtained.

Its thermal conductivity measured at 600 ° C was 2.7 W ° m "1 ° K" 1, and its compressive strength was greater than 100 MPa. Placed on the walls of an oven working at 1450 ° C, this trim proved perfectly stable.

Claims (8)

1. Electrofused corundum refractory material, characterized in that it consists of spheres with a diameter of between 50 μm and 5 mm, with a total porosity rate of between 10 and 50% of the volume of the spheres, the closed porosity. representing from 80 to 98% of the total porosity, wherein the spheres are made of polycrystalline alumina in crystallites less than 50 μηη. Refractory material according to claim 1, characterized in that the total porosity is between 20 and 30% of the volume of the beads. Refractory material according to claim 1 or 2, characterized in that the compressive strength of the spheres of which it is formed is between 150 and 300 g. Refractory material according to any one of claims 1 to 3, characterized in that the thermal conductivity of the bulk material is between 0.5 and 0.8 Wm10K "1. Refractory material according to any one of claims 1 to 4, characterized in that it does not cause any crystalline transformation to reach 1500 ° C. Refractory material according to any one of claims 1 to 5, characterized in that the average diameter of the balls is between 0.5 and 2 mm. Use of materials as defined in any one of claims 1 to 6, characterized in that they are for the manufacture of refractory castables. Use according to claim 7, characterized in that the refractory concrete is laid by projection.